This invention relates generally to the measurement of thread pitch diameters and deals more particularly with a gauge for measuring the pitch diameters on both externally threaded and internally threaded parts.
In the manufacture of threaded parts, the pitch diameter must be measured to assure that the threaded parts have the proper pitch diameter and also in inspection operations to check the pitch diameter of the finished parts. Measurement of the pitch diameter of externally threaded parts can be performed by threaded micrometers which are available in sizes capable of measuring pitch diameters up to three inches. Thread ring gauges and thread plug gauges are go and no-go types of measuring tools having standard sizes for use in measuring standard pitch diameters. Thread pitch wires are also available for use with an outside micrometer, although their use is somewhat cumbersome in that both hands are needed for handling of the thread micrometer and the three wires that are required.
Measurement of the pitch diameter of an internally threaded part is particularly difficult and presents even more of a problem if the threads are specially designed, nonstandard threads. It has been common practice to measure inside pitch diameters by building a special thread plug gauge which is useful for only a single purpose. Delays and significant cost increases are encountered if a large number of nonstandard threads are manufactured.
Special thread gauges have been proposed, as shown by U.S. Pat. No. 2,941,304 to Man, U.S. Pat. No. 4,202,109 to Schasteen, and U.S. Pat. No. 3,111,766 to Bryant. All of these devices are unduly complex and suffer from problems relating to accuracy and versatility. For example, the device of Bryant can measure only external threads, while the Schasteen device requires a complicated dovetail joint and clip arrangement for slidably mounting each of the three gauge assemblies on the blades of a vernier caliper. The gauging rolls shown in the Man patent likewise require complicated clamp devices to attach them to the caliper blades. The Man arrangement also involves the use of helical spring coils for measuring the thread diameter rather than measuring by direct contact of the gauge rolls with the threaded part.